This invention relates generally to digital communication via a high bandwidth (e.g. fiber and coaxial cable) communication medium, and, in particular, to an access protocol and arrangement for allowing competing stations connected to a common head end via a tree and branch communication network to use upstream channels to transmit to the head end, a variety of types of information, such as voice, video-telephony, data or control information that may be synchronous, asynchronous or sporadic. The invention also relates to wireless networks, which are akin to fiber/coax networks, in that mobile stations do not usually directly listen to each other, but instead depend on a base station for feedback.
There is currently a great deal of activity directed toward enhancing the networking infrastructure of cable television companies. The thrust is two-fold: enhancing the downstream capacity of the networks to support new services, and providing for significant upstream capacity for new interactive services, including telephony and data networking. Here, xe2x80x9cupstreamxe2x80x9d refers to transmission from a station to a head end, and xe2x80x9cdownstreamxe2x80x9d refers to transmission from a head end to the stations.
Many current plans for providing upstream capability utilize the well known Time Division Multiple Access (TDMA) method of assigning bandwidth to stations that have invoked a signaling method to indicate to the head-end that they wish to transmit. However, existing versions of TDMA do not provide as much flexibility in the use of the available bandwidth as is desired. Also, packet data techniques do not require the strict allocation of peak bandwidth usually associated with TDMA.
Existing packet data solutions also do not extend well to the broadband cable environment. Carrier Sense Multiple Access/Collision Detection (CSMAJCD), as described, for example in Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, published by the Institute of Electrical and Electronics Engineers, American National Standard ANSI/IEEE Std.
802.3-1985, is arranged so that stations listen to the channel for a period to determine that it is IDLE before transmitting. This type of arrangement is very inefficient in the cable environment, because of the distances involved. The propagation delay, and corresponding dead time of the channel, is much greater in cable networks than in the Local Area Networks that are the standard CSMA/CD environment.
As another example, the slotted Aloha approach described by L. Roberts in xe2x80x9cALOHA Packet System With and Without Slots and Capturexe2x80x9d, Computer Communications Review, April 1975, specifies a system arranged so that time is divided into a series of exact slot periods. Any station that has data may transmit at any time. When collisions occur, stations back off and retransmit according to some randomized algorithm. This approach has also been enhanced with a reservation scheme in which once a station acquires a slot, it can keep successive slots by setting a header field; the station may also signal that it has finished transmitting by changing that field. See, for example, Lam, S. S., Packet Broadcast Networksxe2x80x94A performance analysis of the K-ALOHA Protocol, IEEE Transactions on Computers, Vol. C-29, No. 7, July 1980, 596-603. Here again, the problem is that in the coax/fiber infrastructure under consideration, it is difficult to allow stations to directly listen to each other. Also, the unique tree and branch structure of the cable network requires that a new approach be used.
In accordance with the present invention, a protocol for handling multiple access on broadband fiber/coaxial cable networks is called xe2x80x9cSlotted Occupancy Sense Multiple 20 Access with Centralized Collision Detectionxe2x80x9d or xe2x80x9c505MW for short. SOSMA, which supports both continuous bit rate (CBR) and variable bit rate (VBR) traffic representing voice, video telephony, interactive television, and data. In the case of a coaxial cable or fiber tree and branch communication network, the present invention is carried out both in customer premise equipment (CPE) at stations, and in a common controller, which may be a head end with which all stations communicate. A medium access control (MAC) processor provided in each of the stations and in the common controller or head end ""divides the time domain for a given RF channel into a series of successive frames, each having a plurality of time slots. Because of the architecture of the communication network, individual stations do not communicate directly with each other, but can receive downstream broadcast messages indicating the status of each upstream time slot. These status messages are generated in the common controller or head end, and are transmitted in a broadcast downstream channel so that all stations can receive them. When a station desires to transmit information in the upstream direction, it inserts the information into an xe2x80x98availablexe2x80x99 time slot, with availability being determined in accordance with time slot status. Depending upon the type of traffic being originated, a station can indicate to the common controller or head end a need for continued use of the xe2x80x9csamexe2x80x9d time slot in successive frames. This permits a station, such as a station requiring a CBR connection, to avoid having to contend repeatedly for continued access to the transmission network. In the case of a wireless communication network, the present invention is carried out both in mobile stations, and in a base station which acts as a common controller and with which all mobile stations communicate.
In one embodiment of the invention, acknowledgment or status messages, transmitted in a broadcast downstream channel from the common controller or head end, indicate that a particular time slot status is (a) IDLE, (b) BUSY/CONTINUATION, meaning that a station has a need for continued use of a slot, (c) BUSY/FINAL, meaning that the continued use of a slot has come to an end, or (d) COLLISION, meaning that a collision has occurred with respect to that slot. The IDLE status message occurs when the common controller or head end does not detect any information in a time slot. The BUSY/CONTINUATION message occurs when a transmission was successful, and the station originating the transmission had set the frame continuation bit. The BUSY/FINAL message occurs when a transmission was successful, and the station originating the transmission had set the frame continuation bit to xe2x80x9cfinalxe2x80x9d. The COLLISION status message is formed in the head end by determining that at least one other station connected to the transmission medium may also have attempted a transmission in the same time slot. In a station needing access to one or more time slots on the transmission network, a slot may be considered to be available if the downstream message corresponding to that slot indicates either an IDLE, COLLISION or BUSY/FINAL status.
If a station needs access to more than one time slot in each of a plurality of successive frames, there are several approaches that may be used. First, a station may acquire a first time slot, and then seek second and subsequent time slots by repeating the time slot contention process. In this type of decentralized approach, a station can thus progressively xe2x80x9cramp upxe2x80x9d to obtain the full amount of transmission bandwidth that is needed for the type of traffic being originated in that station. In order to avoid deadlocks, a timer can be used to terminate the call attempt for any station that does not obtain the required number of time slots within a predetermined time period. Alternatively, a station can indicate, in the first upstream signaling message to the head end, that multiple time slots over successive frames are needed. In this type of centralized approach, the head end is arranged to pre-assign the needed slots if they are available, and indicate the assignment in a downstream message.
In accordance with an aspect of the invention, to prevent interference between time slots and to allow ramp up time for transmitters, a guard time is provided between transmit time slots in the upstream direction. However, to increase efficiency, multiple occurrence of the guard times is eliminated in xe2x80x9csuperslots"" that are formed when multiple adjacent time slots in each frame are assigned to a station. These superslots can significantly improve the overall utilization of the available bandwidth on the transmission medium.
In accordance with another aspect of the invention, common controller or head-end based feedback (as opposed to stations listening to the channel for feedback about slot occupancy) can be used to assign time slots and create superslots.
While our invention has thus far been primarily described in the context of a fiber/coax based transmission infrastructure, it is to be understood that the invention is also applicable to a wireless communications environment. In the latter situation, the terms xe2x80x9cmobile stationxe2x80x9d, xe2x80x9cbase stationxe2x80x9d and xe2x80x9cwireless mediumxe2x80x9d, would be applicable in place of the terms xe2x80x9cstationxe2x80x9d, xe2x80x9chead endxe2x80x9d andxe2x80x9d fiber/coax mediumxe2x80x9d.
As a result of the above, a station can autonomously declare its allocated contiguous slots to be used in a superslot mode so as to save bandwidth. This provides for a more efficient, and more economic, use of the channel.